In a fabricating process of a semiconductor device, a plurality of photolithography processes is performed to form patterns on a semiconductor substrate. Particularly, as the integration degree of semiconductor devices increases and a design rule of the patterns formed on the semiconductor substrate decreases, a critical dimension (CD) of the patterns also decreases. For this reason, a photomask with a fine pattern is required to form a highly integrated circuit through the photolithography processes. A binary mask has been generally used as a photomask. In the binary mask, a light blocking pattern is formed on a transparent substrate to irradiate transmitted light on a wafer only through the substrate. Recently, a phase shift mask that can form a finer pattern than the binary mask using a phase shift material having transmittance of several percent is mainly used as the photomask.
FIG. 1 illustrates a cross-sectional view of a rim type photomask as an example of a phase shift mask. Referring to FIG. 1, in a conventional rim type photomask, a mask pattern including a phase shift pattern 105 and a light blocking pattern 110 is disposed on a substrate 100. A groove having a predetermined depth is formed in the substrate 100 to define a phase shift region. The light blocking pattern 110 selectively blocks a portion of the phase shift pattern 105. In the phase shift pattern 105, a region blocked by the light blocking pattern 110 has a height relatively greater than that of the rest region of the phase shift pattern 105. That is, there is formed a height difference between the region blocked by the light blocking pattern 110 and the rest region, thereby defining a rim region 112. The rim region defined by the height difference becomes a high transmittance region which has a light transmittance higher than that of the region which is relatively high in height, that is, the region blocked by the light blocking pattern 110 when the pattern is transferred onto the substrate.
At least two or more lithography processes are required for fabricating such rim type photomask. An exposure process is performed using an electronic beam (e-beam) or a laser having a wavelength ranging from approximately 248 nm to approximately 365 nm in a process of forming the light blocking pattern 100. However, in the case of this exposure unit, it is disadvantageous because the overlay accuracy between the phase shift pattern 105 and the light blocking pattern 110 is approximately 50 nm or more. Therefore, if the overlay accuracy between the patterns 105 and 110 is poor, it is difficult to form a pattern having a size of less than approximately 100 nm. Therefore, since the CD of the pattern decreases, it is further difficult to form the rim type photomask.